1. Related Application
This application is related to a co-pending application entitled "Apparatus For The Resilient Spinwelding of Thermoplastic Articles", Mr. John W. Kelly, R. Frank Smith, and William H. Koacz inventors, filed Apr. 23, 1990, Ser. No. 07/513,381.
2. Field of the Invention
This invention relates to improvements in the spinwelding process wherein plastic workpieces are held together and spun relative to one other so as to cause friction therebetween. The heat generated by the friction creates melt which fuses the workpieces together at their contacting surfaces. Such plastic workpieces may comprise, for example, cups and lids used in the food packaging industry.
3. Description of the Prior Art
Spinwelding is a technique which is used to meld together two thermoplastic workpieces. As is generally known, there are two methods of friction welding, one of which is called the pivot method, the other being the inertial method.
According to the pivot method, the workpieces are aligned axially relative to each other. One workpiece is secured to a rotatable shaft and the other workpiece is held stationary. The front end of the workpiece attached to the rotatable shaft is pressed against the front end of the stationary workpiece and the former workpiece is rotated under axial force. The common surfaces are pressed together and the workpieces are thereby heated by the frictional heat and become plastic. When heated sufficiently to accomplish a weld, a mechanical brake is applied to stop relative rotation between the workpieces.
According to the inertial method (of the present invention), one workpiece is secured to a rotatable shaft on which "rotary inertia rings" are provided. The entire assembly is then rotated up to a desired RPM (revolutions per minute) so as to establish a desired amount of kinetic energy in the rotating assembly. The workpieces are then pressed together and the stored kinetic energy of the assembly is converted into frictional heat at the contact area to soften the contact area and the plastic in the surrouding vicinity. When the rotational energy of the shaft is dissipated, the weld is finished. The rotary inertia rings and initial rate of rotation may be changed to vary the amount of energy input into the weld area.
These two methods are practiced, singly or in combination, in the following U.S. Patents:
U.S. Pat. No. 3,244,574 discloses the provision of means for producing a partially evacuated cavity between two spinwelded workpieces.
U.S. Pat. No. 3,275,179 relates to the preparation of milk bottles and other containers formed from foamed plastic laminate.
U.S. Pat. No. 3,438,825 discloses spinwelding a ring to the surface of an article such as a polyethylene box.
U.S. Pat. No. 3,580,739 discloses welding thermoplastic sheet material by using conical shaped thermoplastic buttons to join the edges of two thermoplastic sheets, as well as a friction drive mountable in a hand drill for spinwelding the thermoplastic buttons to the sheets.
U.S. Pat. No. 3,609,854 discusses a method of removing weld flash after two articles have been spinwelded together.
U.S. Pat. No. 3,690,088 discloses a method of sealably securing a closure element to a flexible tube.
U.S. Pat. No. 3,917,497 discloses the spinwelding of thermoplastic tubing with enlarged end portions to fittings to provide compression type seal joints.
U.S. Pat. No. 3,980,248 discloses a method of fabrication of a bobbin.
U.S. Pat. No. 4,090,898 discloses the use of a gripping member with radially closable jaws contactable with a first thermoplastic workpiece which is mounted upon a second workpiece.
U.S. Pat. No. 4,457,795 discloses a method and apparatus for spin welding together soft and flexible thermoplastic tubes initially held in a telescoping relationship.
And finally, U.S. Pat. No. 4,534,751 discloses a method and apparatus for attaching a thermoplastic container end onto a paper board sidewall, wherein the sidewall is coated with a thermoplastic film, to produce a container.
The above prior art references disclose the use of "hard surfaced" spinning and mounting apparatus, such that there is not mechanical resiliency incorporated into the mandrels, jaws, stationary supports, collets, or other gripping apparatus used to rotate and force one plastic article against another, during the spinwelding process.
A version of the "hard surfaced" apparatus was initially purchased in an unsuccessful attempt to spinweld seven lids to a corresponding number of container flanges, each lid and flange having flat annular mating surfaces. Only one of the spinwelded containers/lids, (sample 55F), was of such quality so as to even warrant burst testing, (the burst test results are given for samples 53A through 48G in the left portion of Table 1, included herein). Sample 55F, however, exhibited a burst pressure of only 20 psig, which is not considered a quality burst pressure for a "robust" weld. A robust weld would have a burst pressure of approximately 50 psig, indicative of the existence of a hermetic seal at the annular mating surfaces.
The desired 50 psig is not driven by a retort pressurization requirement. It is more likely that the achievement of a 20 psi pressure differential across the container/lid boundary would meet retort requirements. However, 50 psig is an indicator that a truly robust and hermetic seal has been made which is just as important. At this pressure, the failure mechanism of the container/lid system is rupture (material failure) of the thin lid, and the weld is stronger than the lid material.
An additional problem was encountered during the spinwelding of these seven samples in that it took approximately 1 second to spinweld each lid to each container flange, a time apparently unacceptable in the area of mass-produced food packaging processes.
A method therefore needs to be developed that would hermetically join a lid to a container in a process time of less than 0.5 seconds, wherein the resultant lidded container would achieve burst pressures of approximately 50 psig in a consistent manner.